>
>>There have been some good suggestions as to grounding. A good Single Point
>>Ground (SPG) is essential.
>
> ------------ REPLY FOLLOWS ------------
>
> I have heard this stated before, but in the case of lightning, I don't
> understand the reason why. I do understand in the case of AC mains
> safety grounding, but not for lightning.
>
> It would seem to me that a strike should be sent to earth as close to
> the point of the strike as possible, rather than routed off many feet
> to an SPG. Could you please explain?
We have some good engineers on here who know much more about this than I,
but...
I guess I'll start at the end and work backwards.
First; yes, you do need a good ground at the tower where a good ground
consists of a series of heavy, bare copper cables CadWelded (TM) or some
other exothermic bond to a number of ground rods spaced roughly twice their
length apart. Although I disagree with some of the information on the
Polyphaser site, they have some very good information on grounding. There
is some disagreement as to how far out the tower can be before you don't
need to tie its ground system into the Single Point Ground (SPG). As the
coax coming in it also a route for the lightning I prefer to run a good
ground ( complete with ground rods) along with the coax to a grounded
bulkhead at the house entrance. That bulkhead is my SPG.
Lightning is often erronously though of as DC, but each strike consists of
multiple strokes with steep rise and fall times. The steeper the rise and
fall times (which aren't necessiarily the same Dv/Dt or Di/Dt) the higher
the fundamental frequency with a wave form rich in harmonics. The current
varies over many orders of magnitude between lightning strikes and this
along with the various rise and fall times varies the effect on nearby
wiring. This is just one of the reasons lightning is so unpredictable.
As I mentioned previously a lightning strike a mile away can induce voltages
as much as 1000 volts (or even more) per meter in wiring. A lightning
strike to a tower and or antenna system 50 or 150 feet from the station
will result in not only high currents and voltages on the feed and control
lines, but very high induced voltages in the house and even between pieces
of disconnected equipment. Again these induced voltages will have relatively
steep rise and fall times however the inductance and capacitance of that
wiring will have an effect (modify) the wave form. Tying the ground systems
togeter *tends* to, or helps to keep the voltage rises from the induced
voltages in the wiring AND grounds close to the same. Although we think of
the house wiring as being the one that raises in potential, so too does the
ground system with a lightning strike. All the voltages in the house are in
reference to the neutral and ground system. If all the grounds are tied
togeter and tend to rise along with the wiring in the house then there is
*relatively* little potential difference between the hot wires and ground
and damage is minimized.
One effect the steep rise and fall times have is to create a lag in the
voltage rise over distance. The higher the frequency (steeper rise time)
the more the difference. If the tower is far enough from the house and the
rise time steep enough the tower can have reached its full potential before
the voltage begins to rise at the house. In other words the connection from
the tower to the house may have little if any effect. The tower no longer
sees the house as part of its ground system even though they are connected.
BUT with the wide range of rise times and tremendous range of power in
lighting strikes it's a good idea to tie them together. For any reasonable
distance it's essential to tie them together.
Most electrical damage is done by energy coming in on the phone lines or AC
mains. As these should have their own grounds where they enter the house,
not connecting those grounds into the SPG allows them to rise to a different
potential. With all the grounds tied together surge protectors have a much
easier time.
Having said all this, there is little we can do for complete protection and
in particular the so called super strikes, but a good SPG *WILL* minimize
the risk and damage.
I hope I phrased this correctly and it's of some help.
73
Roger (K8RI)
>
> Thank you,
>
> Bill W6WRT
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